Environmental pollution caused by combustion-generated NO.sub.x emissions, is a matter of great concern to the public, and as well to industrial fuel users. Beginning in the 1960's, governmental agencies, indeed prompted by public concern with increasing levels of smog and air pollutants, imposed NO.sub.x reduction requirements upon existing power plants in major metropolitan areas. These restrictions were expanded in the 1970's and 1980's to include virtually all industries with combustion equipment. Industry, accepting the challenge, has already developed a large variety of technologies to meet the new needs. Modifying the combustion process has become the most widely used technology for reducing combustion generated NO.sub.x. In addition, a number of flue gas treatment technologies have been developed and are emerging as the primary method of control for certain applications, but have seen limited use where natural gas is the fuel of choice.
Oxides of nitrogen (NO.sub.x) are formed in combustion processes as a result of thermal fixation of nitrogen in the combustion air ("thermal NO.sub.x "), by the conversion of chemically bound nitrogen in the fuel, or through "prompt-NO.sub.x " formation. Thus, in addition to generating "thermal NO.sub.x ", i.e., by high temperature combination of free nitrogen and oxygen, where the fuels employed by such users (e.g. coal gas) contain substantial quantities of chemically bound nitrogen, certain combustion conditions will favor the formation of undesirable NO-type compounds from the fuel-bound nitrogen. "Prompt NO.sub.x " refers to oxides of nitrogen that are formed early in the flame and do not result wholly from the Zeldovich mechanism. Prompt-NO.sub.x formation is caused by 1) interaction between certain hydrocarbon components and nitrogen components and/or, 2) an overabundance of oxygen atoms that leads to early NO.sub.x formation. For natural gas firing, virtually all of the NO.sub.x emissions result from thermal fixation, i.e. "thermal NO.sub.x ", or from prompt NO.sub.x. The formation rate is strongly temperature dependent and generally occurs at temperatures in excess of 1800.degree.K (2800.degree. F.) and generally is more favored in the presence of excess oxygen. At these temperatures, the usually stable nitrogen molecule dissociates to form nitrogen atoms which then react with oxygen atoms and hydroxyl radicals to form, primarily, NO.
In general, NO.sub.x formation can be retarded by reducing the concentrations of nitrogen and oxygen atoms at the peak combustion temperature or by reducing the peak combustion temperature and residence time in the combustion zone. This can be accomplished by using combustion modification techniques such as changing the operating conditions, modifying the burner design, or modifying the combustion system.
Of the combustion modifications noted above, burner design modification is most widely used. Low NO.sub.x burners are generally of the diffusion burning type, designed to reduce flame turbulence, delay the mixing of fuel and air, and establish fuel-rich zones where combustion is initiated. Manufacturers have claimed 40 to 50 percent nominal reductions, but significant differences in the predicted NO.sub.x emissions and those actually achieved have been noted. The underlying cause for these discrepancies is due to the complexity in trying to control the simultaneous heat and mass transfer phenomena along with the reaction kinetics for diffusion burning.
Illustrative of the foregoing and related techniques for NO.sub.x reduction, are the disclosures of the following United States patents:
DeCorso, U.S. Pat. No. 4,787,208 discloses a low-NO.sub.x combustor which is provided with a rich, primary burn zone and a lean secondary burn zone. NO.sub.x formation is inhibited in the rich burn zone by an oxygen deficiency, and in the lean burn zone by a low combustion reaction temperature. Ceramic cylinders are used at certain parts of the combustion chambers.
Furuva et al, U.S. Pat. No. 4,731,989 describes a combustion method for reducing NO.sub.x emissions, wherein catalytic combustion is followed by non-catalytic thermal combustion.
Davis, Jr. et al, U.S. Pat. No. 4,534,165 seeks to minimize NO.sub.x emissions by providing operation with a plurality of catalytic combustion zones and a downstream single "pilot" zone to which fuel is fed, and controlling the flow of fuel so as to stage the fuel supply.
DeCorso, U.S. Pat. No. 4,112,676 shows a combustor generally of the diffusion burning type for a gas turbine engine.
Pillsbury, U.S. Pat. No. 4,726,181 provides combustion in two catalytic stages in an effort to reduce NO.sub.x levels.
Kendall et al, U.S. Pat. No. 4,730,599 discloses a gas-fire radiant tube heating system which employs heterogeneous catalytic combustion and claims low-NO.sub.x catalytic combustion.
Shaw et al, U.S. Pat. No. 4,285,193 describes a gas turbine combustor which seeks to minimize NO.sub.x formation by use of multiple catalysts in series or by use of a combination of non-catalytic and catalytic combustion.
Pfefferle, U.S. Pat. No. 3,846,979 describes low NO.sub.x emissions in a two-stage combustion process wherein combustion takes place above 3300.degree. F., the effluent is quenched, and the effluent is subjected to catalytic oxidation.
Beremand et al, U.S. Pat. No. 4,087,962, discloses a combustor which utilizes a non-adiabatic flame to provide a low emission combustion for gas turbines. The fuel-air mixture is directed through a porous wall, the other side of which serves as a combustion surface. A radiant heat sink is disposed adjacent to the second surface of the burner so as to remove radiant energy produced by the combustion of the fuel-air mixture, and thereby enable operation below the adiabatic temperature. The inventors state that the combustor operates near the stoichiometric mixture ratio, but at a temperature low enough to avoid excessive NO.sub.x emissions. In one embodiment the radiant heat sink comprises a further porous plate.
In U.S. Pat. No. 4,811,555, of which Ronald D. Bell, one of the applicants of the present application, is patentee, there is described a cogeneration system in which NO.sub.x is controlled by the treatment of the turbine exhaust by a combination of combustion in a reducing atmosphere and catalytic oxidation.
In McGill et al, U.S. Pat. No. 4,405,587, for which Ronald D. Bell is a co-patentee, the NO.sub.x content of a waste stream is controlled by treating it and subjecting it to high-temperature combustion in combined reducing and oxidation zones.
Recent work by several of the present co-inventors and others, has resulted in a combustion device which utilizes a highly porous inert media matrix to provide for containment of the combustion reaction within the porous matrix ("PM") --which may comprise fibers, beads, or other material which has a high porosity and a high melting temperature. Preferably, a ceramic foam is used. This ceramic, sponge-like material has a porosity (typically about 90%) which provides a flow path for the combustible mixture. The energy release by the gas phase reactions raises the temperature of the gases flowing through the porous matrix in the postflame zone. In turn, this convectively heats the porous matrix in the postflame zone. Because of the high emissivity of the solid in comparison to a gas, radiation from the high temperature postflame zone serves to heat the preflame zone of the porous material which, in turn, convectively heats the incoming reactants. This heat feedback mechanism results in several interesting characteristics relative to a free-burning flame. These include higher burning rates, higher volumetric energy release rates, and increased flame stability resulting in extension of both the lean and rich flammability limits. In addition to the ability to achieve very high radiant output from a very compact combustor, flame temperature increases are negligible. This is an important consideration with respect to NO.sub.x control purposes.
A one-dimensional mathematical model was constructed that included both radiation and accurate multi-step chemical kinetics. This model was used to predict the flame structure and burning velocity of a premixed flame within an inert, highly porous medium. The various predictions of this model have been discussed by Chen et al. See "The Effect of Radiation on the Structure of Premixed Flames Within a Highly Porous Inert Medium", Y-K Chen, R. D. Matthews, and J. R. Howell, Radiation, Phase Change, Heat Transfer, and Thermal Systems, ed. by Y. Jaluria, V. P. Carey, W. A. Fiveland, and W. Yuen (eds.), ASME Publication HTD-Vol. 81, 1987. "Premixed Combustion in Porous Inert Media"; Y-K Chen, R. D. Matthews, J. R. Howell, Z-H Lu, and P. L. Varghese, Proceedings of the Joint Meeting of the Japanese and Western States Sections of the Combustion Institute, pp. 266-268, 1987; and "Experimental and Theoretical Investigation of Combustion in Porous Inert Media", Y-K Chen, R. D. Matthews, I-G Lim, Z. Lu, J. R. Howell, and S. P. Nichols, Paper PS-201, Twenty-Second Symposium (International) on Combustion, 1988. These papers demonstrate that a porous matrix (PM) combustor can provide a number of advantages over diffusion burners. However, these papers are focused on the development of this new concept, but are not concerned with the problem of NO.sub.x emissions, much less with the effective reduction of same.
The latter issue is, however, addressed in our parent Ser. No. 554,748 application in which low NO.sub.x combustion is effected by a method wherein a fuel, e.g., natural gas, and a source of oxygen, e.g., air, are mixed and the mixture is combusted in at least two successive combustion zones filled with a porous matrix, the void spaces of which provide sites at which substantially all of the said combustion occurs. Preferably, the method utilizes three such combustion zones. The first or most upstream zone is filled with a said porous matrix, and the mixture provided thereto is fuel-lean. In the second successive zone the mixture is fuel-rich; and in the third zone the mixture is fuel-lean.
Ser. No. 670,286, of which this application is a continuation-in-part, addresses a serious problem that has been experienced with PM burners, i.e. flame flashback from the postflame to preflame zones. The latter may include ceramic foam and/or flow mixing and distributing means such as ceramic honeycomb, glass beads or other media, or simply media void mixing space. Flashback of the flame from the postflame zone where combustion is desired, aside from creating potential or actual danger, by definition is uncontrolled burning --which is precisely the condition sought to be avoided in order to preclude or limit NO.sub.x formation. It might be thought that by providing a sufficient rate of fuel/air flow through the PM combustion zone, the problem could be eliminated, i.e. by using a flow rate exceeding the possible rate of back propagation of the flame. It develops, however, that in the real system present in the PM burner, the porous media, as for example where same is in the general shape of a solid cylinder, acts with respect to the normally axial flow of the fuel-air mixture through such cylinder, to cause an uneven rate of flow across a plane transverse to the cylinder. Specifically, there will tend to be flow stagnation at the peripheral walls of the cylinder, as opposed to the generally maximum flow rate occurring at the axis. Accordingly, merely increasing the rate of flow of the fuel-air mixture is not generally sufficient to assure the absence of undesired flame flashback to the preflame zone.
The problem presented by the foregoing is recognized in Fleming, U.S. Pat. No. 4,643,667. In this, Fleming discloses a noncatalytic porous phase combustor comprising a porous plate having at least two discrete and contiguous layers, a first preheat layer comprising a material having a low inherent thermal conductivity, and a second combustion layer comprising a material having a high inherent thermal conductivity and also providing a radiating surface. The presence of the low conductivity material tends to limit the heating in that initial zone, thereby discouraging flashback. The construction recommended by Fleming is, however, a very complex and difficult one to achieve. Furthermore, the presence of the contiguous low conductivity material, while affording advantages as aforementioned, also introduces a pressure drop into the flow, with no commensurate benefits.
In the apparatus of the Ser. No. 670,286 invention, mixing and flow directing means are provided for receiving and mixing a fuel, e.g. natural gas, and a source of oxygen, e.g. air, and forming a flow of the combustible mixture. The combustible mixture is flowed downstream to a combustion zone defined by a porous high temperature-resistant matrix, the void spaces of which provide sites at which substantially all of the combustion occurs, which zone includes an input end for receiving the combustible flow from the mixing and flow directing means. Cooling means are mounted in proximity to the input end of the combustion zone for maintaining the temperature of the combustible mixture at the input end below ignition temperature, to thereby limit the flame produced by combustion in the porous matrix to the downstream or postflame side of the cooling means. The cooling means typically comprises a generally toroidal metal body which is provided with one or more internal cooling channels. This body surrounds, and is in thermal contact with the input end of the combustion zone. Means are provided for circulating a coolant through the body, which coolant can typically be water but may be other liquid media or a gas, including air. The cooling body is so mounted as to be nonintrusive with respect to the porous matrix in the combustion zone, so as to introduce no impedance to the flowing fuel and oxygen source mixture.
The Ser. No. 670,286 invention is applicable to a single stage porous matrix burner, as well as to the multiple stage devices which are disclosed in parent application Ser. No. 554,748. In any of these instances, the cooling means is positioned as to be at the input end (i.e. in advance) of the first (or single) stage whereat combustion is to be effected. The cooling stage in each instance acts to produce a sharp discontinuity in temperature so that even where the flow stagnation effect aforementioned (which tends to occur at the periphery of the porous matrices) is present, there is substantially no danger of flashback from the flame of combustion which exists in the postflame PM zone(s). By eliminating the flashback potential, it is found that extremely stable, well-formed flames result, which in turn provide the highly controlled combustion conditions which are one of the objectives sought after in porous media burners, for the special objective of reducing generation of NO.sub.x.
A combustion process is thus provided enabling controlled low NO.sub.x combustion. Fuel and an oxygen source such as air are mixed and formed into a combustible flow stream. The flow stream is passed to an input end of a combustion zone defined by a porous high temperature-resistant matrix. The mixture is combusted at the matrix, the void spaces of which provide sites at which substantially all of the said combustion occurs, and the combustion products are flowed from an output end of the matrix. The input end of the combustion zone is cooled, to maintain the temperature of the combustible mixture at the said input end below ignition temperature, thereby limiting the flame produced by combustion in the porous matrix to the downstream side of the cooling means.